The invention relates generally to the field of optical lasers, and more particularly to the tuning of wavelength emissions from a laser.
Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of 50 GHz, or approximately 0.4 nanometers. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in DWDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The trend towards smaller channel separation and the advent of channel selectivity in transmitter lasers has given rise to a need for greater accuracy and control in the positioning of tunable elements associated with transmitter lasers. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. Non-optimal positioning of tunable elements results in spatial losses and reduced transmitter output power.
The use of an air-spaced etalon for the tuning of external cavity lasers has been attempted. Such an etalon is constructed from two pieces of glass each having a high-reflectance (HR) coated surface and an anti-reflection (AR) coated surface. The high reflectance surfaces of the two pieces are placed facing one another and are separated by a spacer. The spacer is formed using tiny drops of epoxy filled with precision glass beads. This type of etalon is not dimensionally stable, even over short time spans, making it difficult to use the laser for any extended period of time without recalibrating it.
The invention relates to tunable external cavity lasers and improvements in tuning elements therefor. The invention comprises, in general terms, a tunable laser configured to use a graded thin film interference filter as a channel selector. An external cavity laser according to the present invention incorporates a gain medium and a tapered, thin film interference filter. The external cavity laser may additionally comprise an external reflective element, a grid generator element and a collimator. The grid generator element, collimator and tapered, thin film interference filter may be aligned in an optical path between the gain medium and the external reflective element.
A driver may be operably connected to the tapered, thin film interference filter and adapted to adjustably position it in the optical path. One method of adjusting the tuning element or tuning the laser includes linearly translating the thin film interference filter in directions normal to the optical path, although other methods of tuning include rotating a circular variable filter in the optical path, or tilting a filter in the optical path.
The tapered, thin film interference filter according to the invention may comprise a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of said half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of said half wave spacer layer.
A tapered, thin film interference filter according to the invention may include more than one spacer layer. Each spacer layer, whether one or more is employed, has an optical wavelength which is an odd integral multiple of a half wavelength of light to be passed therethrough, and each side thereof is adjacent a tapered layer stack.
A tuning element for an external cavity laser is disclosed, which includes a tapered thin film interference filter including a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of the half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of the half wave spacer layer.
A method for tuning an external cavity laser according to the invention may include providing a tapered thin film interference filter including a tapered half wave spacer layer, a tapered first quarter wave layer stack positioned adjacent a first side of the half wave spacer layer, and a tapered second quarter wave layer stack positioned adjacent a second side of the half wave spacer layer, and adjustably positioning the tapered thin film interference filter in an optical path defined by a beam associated with the external cavity laser.
Adjustment of the position may include translating the thin film interference filter in at least one direction substantially normal to the optical path. Alternatively, adjustment of the positioning of the filter may include rotating the thin film interference filter in a plane substantially normal to the optical path, or tilting the thin film interference filter by rotating it about an axis substantially normal to the optical path.
A method for tuning an external cavity laser is disclosed to include generating a first table of position locations, at a baseline temperature, to move a tuning element to respective desired wavelengths to be emitted from the external cavity laser, storing data values representative of the first table in non-volatile memory; generating a second table of position location adjustments to be made relative to a range of variations in temperature surrounding the baseline temperature, storing data values representative of the second table in non-volatile memory; obtaining a gross position of the tuning element in accordance with a wavelength desired to be emitted from the laser by accessing the first table and selecting the position location stored for the desired wavelength; measuring a temperature of the environment surrounding the tuning element; obtaining a position location adjustment value from the second table that corresponds to the temperature measured; and determining position correction to finely adjust the gross position, and moving the tuning element to the finely adjusted position, thereby accurately tuning the laser to emit the desired wavelength.
A method of mounting a tuning filter or etalon in a substantially stress free manner is disclosed to include the provision of a rigid mount having a multiplicity of pads, including a central pad mounted thereon, wherein the central pad has a recessed support surface, relative to support surfaces defined on a remainder of the multiplicity of pads. A substantially non-shrinking adhesive is applied on the recessed support surface, wherein the adhesive builds to the height of the central pad to be substantially equal to height of the support surfaces on the remainder of the multiplicity of pads. The etalon is then mounted by placing a bottom end of the etalon on the pads, whereby the adhesive bonds to a central portion of the end of the etalon.